1. Field of the Invention
The present invention relates to a transmitter tube of the type designed to transmit radio frequency (RF) radiation, including microwaves, and in particular to a high power transmitter tube in which beam control pulse modulation is accomplished by (1) connecting the control electrode directly to the cathode power supply, and (2) connecting a solid state switch between the cathode power supply and the cathode of the transmitter tube. Closing of the solid state switch brings the beam controlling element voltage to cathode potential, permitting an electrode beam emitted by the cathode to reach a collecting electrode or electrodes of the transmitter tube, while opening of the switch provides a very high equivalent cathode resistance, which self-biases the tube in the cutoff region and cuts off the beam to form a pulse. The invention also relates to a method of beam control pulse modulation for a transmitter tube, in which the beam controlling element is returned to the cathode power supply and a solid state switch is connected between the cathode power supply and the cathode of the transmitter tube.
2. Description of Related Art
Transmitter tubes are currently used in a wide variety of applications, including satellite communications, electronic countermeasures, radar transmitters, medical x-ray source control, and so forth. Most transmitter tubes consist of a cathode which emits electrons when the energy of the surface atoms of the cathode is raised, and one or more electrodes for collecting the emitted electrons and for establishing variable electric fields in order to control the movement of the electrons between the emitting electrode and the collecting electrode or electrodes. While magnetic fields may be used to control movements of the electrons being emitted by the cathode, control by electric fields has a number of advantages and is the basis for the type of transmitter tube to which the present invention is directed, in which the primary source of electrons is thermionic emission from the cathode and control is by electric fields generated by at least one control electrode.
Those skilled in the art will appreciate, however, that even though the invention is described in terms of a particular type of transmitter tube, it is adaptable for use with a variety of different tube configurations, and that the description herein of a particular type of transmitter tube in connection with a preferred embodiment is to be taken as illustrative rather than limiting.
Specifically, the preferred embodiment of the invention is described in terms of a type of thermionic vacuum tube known as a travelling wave tube (TWT), commonly used for microwave frequency transmissions in radar systems, and in which the control electrode is known as the "focusing electrode." Other transmitter tubes use a type of control electrode known as a "grid." As will become apparent from the detailed description, the invention is equally applicable to transmitter tubes having control electrodes in the form of focusing electrodes, grids, or any combination thereof, and may also be applicable to other types of tubes, so long as a control electrode and cathode are present.
Whether the control electrode is in the form of a grid or focusing electrode, the purpose of the control electrode is to generate an electric field which modulates the amount or direction of electrons emitted by the cathode, with the strength of the control field relative to the cathode field and the energy of the electrons emitted by the cathode determining how many of the emitted electrons reach the collecting electrode(s) or anode. In a typical arrangement in which the control electrode is adjacent the cathode, for example, when the control electrode is at the cathode potential, the control electrode will have little or no effect on the electrons emitted by the cathode. On the other hand, if the voltage of the control electrode is sufficiently large relative to the cathode voltage, all or most electrons will deflected or prevented from leaving the cathode, and the electron beam will be effectively cut-off. This potential difference between the control electrode and the cathode is known as the tube cutoff voltage.
In a pulse modulated transmitter tube, the desired output is in the form of pulses which are conveniently generated by switching the voltage of the control electrode between the cathode potential and the cutoff voltage. In a high power transmitter tube, such as the travelling wave tube whose characteristics are illustrated in FIG. 2, the cathode is at a very high negative voltage, which typically ranges from 2000 Vdc to 20,000 Vdc, so as to provide sufficient energy to accelerate the emitted electrons. In FIG. 2, the line indicated by solid squares (10) represents the current (Ik) emitted by the cathode as a function of the cathode to control electrode (Vfe) potential for a conventional travelling wave transmitter tube, in which the cathode voltage is at -3700 V (the open square line (20) represents an application of the present invention and will be discussed below). As is apparent from this FIGURE, the potential difference between the control electrode and the cathode in the conventional transmitter tube must be approximately 1280 volts in order to bring the cathode current (Ik) to zero.
Control of the conventional transmitter thus requires, as illustrated in FIG. 1, a power source 1 of -3700 volts for the cathode 2, a control electrode power source 7 for establishing a cutoff voltage of -1280 volts by supplying the cutoff voltage to the control electrode 4, an ON switch 5 for connecting the control electrode 4 to the cathode high voltage power source (HVPS) 1 for decreasing the control electrode to cathode potential to zero in order to permit emission of electrons from the cathode (see the point (0, 180) in FIG. 2), and an OFF switch 6 for connecting the control electrode 4 to the control electrode power source 7 in order to raise the control electrode 4 to the cutoff potential and switch off the beam of electrons being emitted by the cathode 2 to terminate a pulse. Pulse modulation is thus achieved by toggling of either the OFF or ON switches 5 and 6 in order to activate and deactivate the beam current of the transmitter tube 8 and thereby generate a pulse train.
Although the above-described conventional type of transmitter tube pulse modulation can provide an RF transmitter tube with adequate beam current control, and is widely used in both military and civilian applications, the conventional arrangement has a number of shortcomings, which those skilled in the art have heretofore been unable to solve. These include:
1. The circuit complexity resulting from the high voltages at which conventional transmitter tubes of the type illustrated in FIGS. 1 and 2 operate in turn results in increased weight and higher volume packaging, which is especially critical in the case of airborne equipment such as airborne radar transmitters;
2. The conventional circuitry operates at an increased stress level which reduces overall reliability;
3. Conventional control arrangements for the control electrode result in an appreciable increase in tube cut-off voltage; and
4. As illustrated in FIG. 1, conventional transmitter tubes require an additional off-bias voltage power supply, which add to both the weight and volume of the transmitter packaging.
It is known to control the cathode of other types of tubes by modulating the cathode potential as disclosed, for example, in U.S. Pat. No. 5,038,051, directed to a circuit which replaces a tetrode switch tube for modulating a cross field amplifier of a microwave transmitter tube. However, this type of cathode control is not analogous to the cathode control provided by the present invention, but rather involves switching the cathode voltage completely on and off, which in the example illustrated by FIG. 2 would involve switching the cathode between a voltage of 0 V and -3700 V. The solution offered by this patent in fact involves a relatively complicated circuit due to the high voltages involved. In contrast, as explained in more detail below, the present invention involves switching the cathode potential by an amount approximately equal to the cut-off potential, which in the example shown in FIG. 2, is approximately -1280 V for control electrode modulation and even less for the cathode modulation of a preferred embodiment of the invention (for reasons to be explained below). The present invention, which involves directly connecting the control electrode of a transmitter tube to the cathode power supply and selectively connecting the cathode to its power supply to reduce the potential difference between the cathode and the control electrode from the cut-off voltage to zero, is therefore completely different from the type of circuitry disclosed in U.S. Pat. No. 5,038,051, as well as from the type of control electrode modulated transmitter tube arrangement illustrated in FIGS. 1 and 2 and also disclosed, for example, in U.S. Pat. Nos. 4,728,809 and 4,877,996.